1. Field of the Invention
The present invention relates generally to a reverse rate control apparatus and method in a mobile communication system, and in particular, to an apparatus and method for transmitting a control signal for reverse traffic.
2. Description of the Related Art
In a mobile communication system, data transmission is divided into forward data transmission and reverse data transmission. Forward data transmission refers to data transmission from a base station system (BSS) to a mobile station (MS), and reverse data transmission refers to data transmission from the mobile station to the base station system. According to the types of transmission data supported, the mobile communication system can be classified into a system supporting only a voice service, a system supporting a voice service and a simple data service, a system supporting only a high-speed data service, and a system supporting both a high-speed data service and a voice service.
The mobile communication system providing both the high-speed data service and the voice service supports a multimedia service using the same frequency band. In this mobile communication system, a plurality of users simultaneously transmit data by a Code Division Multiple Access (CDMA) scheme. In the CDMA scheme, user identification is achieved through unique numbers assigned to the users. In the CDMA scheme, reverse data transmission is achieved through a packet data channel on a per-physical layer packet (PLP) basis, and a length of the packet is fixed according to a data rate. A data rate of the packet is variable for each packet, and a rate of each packet is controlled by the power and the amount of transmission data of a mobile station and a power control bit (PCB). The power control bit is control information transmitted from a base station system over a rate control channel (RCCH).
In addition, the mobile communication system enables retransmission in a physical layer in order to improve reverse throughput. For retransmission in a physical layer, a base station system demodulates a received reverse data packet, and transmits an ACK/NACK of a physical layer according to whether a packet error, or a packet error, exists in the received reverse data packet.
Upon receiving ACK from the base station system, a mobile station transmits a new packet, determining that the transmitted packet has been successfully received at the base station system. However, if NACK is received from the base station system, the mobile station retransmits the previous packet, determining that the transmitted packet has not been successfully received at the base station system.
A function of determining a data rate of a mobile station is called “scheduling.” A data rate of a mobile station is determined by scheduling. A base station system performs the scheduling using Rise over Thermal (RoT), which indicates a ratio of thermal nose to total reception power, or a load obtained from a received signal to nose ratio of a mobile station belonging to a current base station system. When RoT is available, the base station system performs scheduling using measured RoT, assigned load, and available remaining capacity. However, when RoT is unavailable, the base station system measures a load, and performs scheduling by taking the measured load and available remaining capacity into account.
That is, a scheduler of the base station system determines whether to increase, decrease, or hold a data rate of a corresponding mobile station, taking RoT, a buffer status of each mobile station, and power status or channel condition of each mobile station into account.
The base station system can increase the throughput of the system by efficiently controlling a data rate of a mobile station in this manner. The data rate information of a mobile station is transmitted to a particular mobile station through a rate control bit (RCB). A value of the rate control bit transmitted from the base station system is ‘0’, ‘+1’, or ‘−1’. If a value of the rate control bit is ‘+1’, the mobile station increases its data rate by one step in a next transmission period. If a value of the received rate control bit is ‘−1’, the mobile station decreases its data rate by one step in the next transmission period. If a value of the received rate control bit is ‘0’, the mobile station holds its data rate in the next transmission period.
The scheduler can be included in a base station system (BSS) or a base station controller (BSC) according to system design. In the following description, first “BSS scheduling” will be described, and then “BSC scheduling” will be described.
Generally, a mobile station manages an active set. The “active set” refers to a list of base station systems communicating with the mobile station. For example, while a particular mobile station is performing a handoff, information on several base station systems in communication with the corresponding mobile station is included in an active set of the corresponding mobile station. While performing handoff, a corresponding mobile station receives rate control bits from several base station systems included in its own active set as a result of scheduling. In a system where BSS scheduling is performed, rate control bits received at the mobile station from the base station systems in the active set are different from one another. In a system where BSC scheduling is performed, rate control bits received at the mobile station from the base station systems in the active set are identical to one another.
Some advantages and disadvantages of the BSS scheduling scheme and the BSC scheduling scheme will be described herein below.
In the BSC scheduling scheme, combining is available when receiving rate control bits of a mobile station. Therefore, reception reliability of rate control bits is increased, and the base station system can transmit rate control bits with lower power. However, a scheduling delay time is increased.
“Combining” refers to combining received signals and demodulating the combined signal in a physical layer. Further, the “scheduling delay time” refers to a time required when a mobile station transmits its status information (e.g., buffer capacity or power status) in a reverse direction to undergo scheduling. A base station system receives the status information of the mobile station, performs scheduling using the received status information together with other information necessary for scheduling and transmits the scheduling result to the mobile station. The mobile station receives the scheduling result and applies the received scheduling result.
In the BSS scheduling scheme, the scheduling delay time is short. However, combining is unavailable when receiving rate control bits of a mobile station, thereby causing a decrease in reception reliability of rate control bits. As a result, a base station system should transmit rate control bits with higher power.
A mobile communication system allows retransmission in a physical layer in order to improve reverse throughput. The retransmission in a physical layer includes a base station system demodulating a received reverse data packet and transmitting ACK/NACK of a physical layer according to whether a packet error (or CRC error) exists in the received reverse data packet, and a mobile station receiving the ACK/NACK and determining whether it will retransmit previously transmitted packet or transmit new packet. The retransmission procedure by the physical layer is called “Hybrid Automatic Repeat and Request (HARQ).” In a system supporting the HARQ, when the ACK/NACK bit is generated by a BSS, it is called “BSS transmission,” and when the ACK/NACK bit is generated by a BSC, it is called “BSC transmission.”
In a system in which BSS transmission is performed, ACK/NACK bits received at a mobile station from base station systems can be different from one another. However, in a system in which BSC transmission is performed, ACK/NACK bits received at the mobile station from the base station systems in the active set will be identical to one another.
As indicated above, in the BSS transmission scheme, a retransmission delay time is reduced. However, combing is unavailable when receiving ACK/NACK bits for a mobile station, thereby causing a reduction in reliability of ACK/NACK bits. In the BSC transmission scheme, combing is available when receiving ACK/NACK bits for a mobile station, thereby causing an increase in reliability of ACK/NACK bits. However, a retransmission delay time is increased. The “retransmission delay time” refers to a time required when a mobile station transmits a packet, a base station system receives the packet, determines whether an error exists in the received packet and transmits an ACK/NACK bit to the mobile station, and the mobile station receives the ACK/NACK bit and transmits a next packet.
According to the BSS scheduling scheme and the BSC scheduling scheme, the mobile station cannot determine a base station system from which rate control bits received can be combined. Therefore, if the mobile station, without knowing the information, receives rate control bits transmitted from all base station systems in its active set, it should always perform independent reception and demodulation process on each of the received rate control bits, thereby decreasing reception reliability of the rate control bits. In order to increase reception reliability of the rate control bits, the base station system should transmit the rate control bits with higher power.
In addition, according to the BSS transmission scheme and the BSC transmission scheme, the mobile station cannot determine a base station system from which ACK/NACK bits received can be combined. Therefore, if the mobile station, without knowing the information, receives ACK/NACK bits transmitted from all base station systems in its active set, it should always perform independent reception and demodulation process on each of the received ACK/NACK bits, thereby undesirably decreasing reception reliability of the ACK/NACK bits.